There are special characteristics for glue which is used in a system for making corrugated cardboard. Among other things, these characteristics include a gelatinization of starch in order to obtaining a desired gel point at a faster rate. The starch gelatinizes in response to a certain quantity of NaOH (Lye) which is introduced into a compound that includes at least water and starch. With the development of machines that produce corrugated board at speeds that are much higher than previous speeds, much higher demands are being placed on the glue or adhesive that is used and the speed at which such glue or adhesive is produced.
Currently, most starch-based glues or adhesives and their production are based on the "Stein-Hall" patent (U.S. Pat. No. 2,102,937). These adhesives are based on:
a gelatinized starch carrier which is used at a viscosity that keeps the secondary starch in suspension and which determines the rheology, the structure, and the visco-stability of the glue. These characteristics greatly influence the amount of glue that is applied on the glue rolls and thus on the paper in the corrugator. PA1 uncooked starch (raw starch) that gelatinized in situ at the corrugator machine results in a rapid gelatinization and formation of a "wet tack, green bond" between the corrugated medium and the liner. PA1 alkaline reagents, commonly caustic soda, assist in the gelatinization of the carrier starch and a lowering of the gelatinization temperature of the uncooked starch. This speeds up the gelatinization "in situ" of the raw starch. PA1 "borax", increases the viscosity (wet tack, green bond) when the uncooked starch is gelatinized on the corrugator. PA1 the amount of caustic soda is increased in order to decrease the gelatinization temperature of the uncooked starch. This results in a faster gelatinization and the formation of a bond between the medium and the liner. PA1 the amount of uncooked starch is increased in order to decrease the amount of water that has to be evaporated from the board. The board is then dry with less time and energy. PA1 make a slurry of the carrier starch which is 10 to 17% dry solid PA1 heat to 30-55.degree. C. PA1 and add caustic soda PA1 stir until the viscosity is stable PA1 dilute with water PA1 add raw starch PA1 add borax PA1 stir until the viscosity is stable
An example of currently used formulations contain 80-77% water, 2-3% gelatinized starch, 25-18% raw starch, 0.1-0.5% caustic soda, and 0.05-0.5% borax.
The gelatinized starch can be unmodified maize (corn), wheat, tapioca or potato starch. Or it is a modified crosslinked starch which enhances the rheological characteristics of the glue that improve its runnability on the machine.
The raw starch can be native or modified crosslinked maize, wheat, potato, or tapioca. The best suited starch has a natural low gelatinization temperature and a high viscosity when gelatinized "in situ" on the corrugator. When this happens, a minimal amount of chemicals (caustic soda and borax) is needed, especially borax, since boron is toxic.
To increase the production speed of the corrugated board machine, a faster adhesive formulation can be prepared. The "Stein-Hall" adhesive can be modified in two ways:
Currently, most "Stein-Hall" adhesives are prepared in a one tank system in the corrugated board factory.
A sequence for making such an adhesive is:
Now formulas are used with a higher amount of caustic soda (lye) and higher amount of raw starch in order to increase the dry solids and to increase the binding speed of the adhesive. The concentration of the caustic soda in the slurry is so high that, when the secondary starch is added into the slurry, the first part of the raw starch partial gelatinizes. This also happens when a gelatinized carrier starch is added to a slurry of raw starch, which this causes an increase in viscosity and deteriorates the viscosity stability of the glue. It also limits the maximum concentration of dry solids in the glue. More important, a premature gelatinization tends to limit the amount of lye that can be added.
A method for the batch preparation of a glue for corrugated board is known from European patent application No. 0 391 477 which provides an excellent result. However, since lye is added at one time, the rapid gelatinization limits the amount of lye that can be introduced into the carrier.
According to Example 1 of this European patent application, the carrier is prepared by adding water, starch, and a NaOH-solution to a first, relatively large, mixing vessel. The contents of the first, relatively large mixing vessel, are circulated from and to the first mixing vessel via a recirculation pipe, in which a second and comparatively smaller mixing vessel is incorporated. The smaller vessel is also provided with means for exerting a shearing force upon the mixture. The shearing forces are much greater in the smaller vessel than in the large mixing vessels. In the second step, water, starch, and powdered borax are supplied to the carrier, which has been prepared in the first mixing vessel, with continuous stirring and recirculating.
Further research has shown that it is possible to improve the properties of the glue which is thus prepared. In particular, is possible to prepare a homogeneous glue. An inhomogeneity may be eliminated by a prolonged circulation of the mixture via the recirculation pipe and the relatively small mixing vessel which is present therein.